Internet Engineering Task Force (IETF) B. Carpenter
Request for Comments: 7136 Univ. of Auckland
Updates: 4291 S. Jiang
Category: Standards Track Huawei Technologies Co., Ltd
ISSN: 2070-1721 February 2014
Significance of IPv6 Interface Identifiers
Abstract
The IPv6 addressing architecture includes a unicast interface
identifier that is used in the creation of many IPv6 addresses.
Interface identifiers are formed by a variety of methods. This
document clarifies that the bits in an interface identifier have no
meaning and that the entire identifier should be treated as an opaque
value. In particular, RFC 4291 defines a method by which the
Universal and Group bits of an IEEE link-layer address are mapped
into an IPv6 unicast interface identifier. This document clarifies
that those two bits are significant only in the process of deriving
interface identifiers from an IEEE link-layer address, and it updates
RFC 4291 accordingly.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7136.
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RFC 7136 IPv6 IID Significance February 2014
The Modified EUI-64 format preserves the information provided by two
particular bits in the MAC address:
o The "u/l" bit in a MAC address [IEEE802] is set to 0 to indicate
universal scope (implying uniqueness) or to 1 to indicate local
scope (without implying uniqueness). In an IID formed from a MAC
address, this bit is simply known as the "u" bit and its value is
inverted, i.e., 1 for universal scope and 0 for local scope.
According to [RFC4291] and [RFC7042], the reason for this was to
make it easier for network operators to manually configure
local-scope IIDs.
In an IID, this bit is in position 6, i.e., position 70 in the
complete IPv6 address (when counting from 0).
o The "i/g" bit in a MAC address is set to 1 to indicate group
addressing (link-layer multicast). The value of this bit is
preserved in an IID, where it is known as the "g" bit.
In an IID, this bit is in position 7, i.e., position 71 in the
complete IPv6 address (when counting from 0).
This document discusses problems observed with the "u" and "g" bits
as a result of the above requirements and the fact that various other
methods of forming an IID have been defined independently of the
method described in Appendix A of RFC 4291. It then discusses the
usefulness of these two bits and the significance of the bits in an
IID in general. Finally, it updates RFC 4291 accordingly.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Problem Statement
In addition to IIDs formed from IEEE EUI-64 addresses, various new
forms of IIDs have been defined, including temporary addresses
[RFC4941], Cryptographically Generated Addresses (CGAs) [RFC3972]
[RFC4982], Hash-Based Addresses (HBAs) [RFC5535], and ISATAP
addresses [RFC5214]. Other methods have been proposed, such as
stable privacy addresses [IID-SLAAC] and mapped addresses for 4rd
[SOFTWR-4RD]. In each case, the question of how to set the "u" and
"g" bits has to be decided. For example, RFC 3972 specifies that
they are both zero in CGAs, and RFC 4982 describes them as if they
were reserved bits. The same applies to HBAs. On the other hand,
RFC 4941 specifies that "u" must be zero but leaves "g" variable.
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RFC 7136 IPv6 IID Significance February 2014
The NAT64 addressing format [RFC6052] sets the whole byte containing
"u" and "g" to zero.
Another case where the "u" and "g" bits are specified is in the
Reserved IPv6 Subnet Anycast Address format [RFC2526], which states
that "for interface identifiers in EUI-64 format, the universal/local
bit in the interface identifier MUST be set to 0" (i.e., local) and
the "g" bit is required to be set to 1. However, the text neither
states nor implies any semantics for these bits in anycast addresses.
A common operational practice for well-known servers is to manually
assign a small number as the IID, in which case "u" and "g" are both
zero.
These cases illustrate that the statement quoted above from RFC 4291
requiring "Modified EUI-64 format" is inapplicable when applied to
forms of IID that are not in fact based on an underlying EUI-64
address. In practice, the IETF has chosen to assign some 64-bit IIDs
that have nothing to do with EUI-64.
A particular case is that of /127 prefixes for point-to-point links
between routers, as standardised by [RFC6164]. The addresses on
these links are undoubtedly global unicast addresses, but they do not
have a 64-bit IID. The bits in the positions named "u" and "g" in
such an IID have no special significance and their values are not
specified.
Each time a new IID format is proposed, the question arises whether
these bits have any meaning. Section 2.2.1 of [RFC7042] discusses
the mechanics of the bit allocations but does not explain the purpose
or usefulness of these bits in an IID. There is an IANA registry for
reserved IID values [RFC5453], but again there is no explanation of
the purpose of the "u" and "g" bits.
There was a presumption when IPv6 was designed and the IID format was
first specified that a universally unique IID might prove to be very
useful, for example to contribute to solving the multihoming problem.
Indeed, the addressing architecture [RFC4291] states this explicitly:
The use of the universal/local bit in the Modified EUI-64 format
identifier is to allow development of future technology that can
take advantage of interface identifiers with universal scope.
However, so far, this has not proved to be the case. Also, there is
evidence from the field that MAC addresses with universal scope are
sometimes assigned to multiple MAC interfaces. There are recurrent
reports of manufacturers assigning the same MAC address to multiple
devices, and significant reuse of the same virtual MAC address is
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RFC 7136 IPv6 IID Significance February 2014
reported in virtual machine environments. Once transformed into IID
format (with "u" = 1), these identifiers would purport to be
universally unique but would in fact be ambiguous. This has no known
harmful effect as long as the replicated MAC addresses and IIDs are
used on different layer 2 links. If they are used on the same link,
of course there will be a problem, very likely interfering with
link-layer transmission. If not, the problem will be detected by
duplicate address detection [RFC4862] [RFC6775], but such an error
can usually only be resolved by human intervention.
The conclusion from this is that the "u" bit is not a reliable
indicator of universal uniqueness.
We note that Identifier-Locator Network Protocol (ILNP), a
multihoming solution that might be expected to benefit from
universally unique IIDs in modified EUI-64 format, does not in fact
rely on them. ILNP uses its own format defined as a Node Identifier
[RFC6741]. ILNP has the constraint that a given Node Identifier must
be unique within the context of a given Locator (i.e., within a
single given IPv6 subnetwork). As we have just shown, the state of
the "u" bit does not in any way guarantee such uniqueness, but
duplicate address detection is available.
Thus, we can conclude that the value of the "u" bit in IIDs has no
particular meaning. In the case of an IID created from a MAC address
according to RFC 4291, its value is determined by the MAC address,
but that is all.
An IPv6 IID should not be created from a MAC group address, so the
"g" bit will normally be zero. But, this value also has no
particular meaning. Additionally, the "u" and the "g" bits are both
meaningless in the format of an IPv6 multicast group ID [RFC3306]
[RFC3307].
None of the above implies that there is a problem with using the "u"
and "g" bits in MAC addresses as part of the process of generating
IIDs from MAC addresses, or with specifying their values in other
methods of generating IIDs. What it does imply is that after an IID
is generated by any method, no reliable deductions can be made from
the state of the "u" and "g" bits; in other words, these bits have no
useful semantics in an IID.
Once this is recognised, we can avoid the problematic confusion
caused by these bits each time that a new form of IID is proposed.
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RFC 7136 IPv6 IID Significance February 20143. Usefulness of the U and G Bits
Given that the "u" and "g" bits do not have a reliable meaning in an
IID, it is relevant to consider what usefulness they do have.
If an IID is known or guessed to have been created according to
[RFC4291], it could be transformed back into a MAC address. This can
be very helpful during operational fault diagnosis. For that reason,
mapping the IEEE "u" and "g" bits into the IID has operational
usefulness. However, it should be stressed that an IID with "u" = 1
and "g" = 0 might not be formed from a MAC address; on the contrary,
it might equally result from another method. With other methods,
there is no reverse transformation available.
Given that the values of the "u" and "g" bits in an IID have no
particular meaning, new methods of IID formation are at liberty to
use them as they wish, for example, as additional pseudo-random bits
to reduce the chances of duplicate IIDs.
4. The Role of Duplicate Address Detection
As mentioned above, Duplicate Address Detection (DAD) [RFC4862] is
able to detect any case where a collision of two IIDs on the same
link leads to a duplicated IPv6 address. The scope of DAD may be
extended to a set of links by a DAD proxy [RFC6957] or by Neighbor
Discovery Optimization [RFC6775]. Since DAD is mandatory for all
nodes, there will be almost no case in which an IID collision,
however unlikely it may be, is not detected. It is out of scope of
most existing specifications to define the recovery action after a
DAD failure, which is an implementation issue. If a manually created
IID, or an IID derived from a MAC address according to RFC 4291,
leads to a DAD failure, human intervention will most likely be
required. However, as mentioned above, some methods of IID formation
might produce IID values with "u" = 1 and "g" = 0 that are not based
on a MAC address. With very low probability, such a value might
collide with an IID based on a MAC address.
As stated in RFC 4862:
On the other hand, if the duplicate link-local address is not
formed from an interface identifier based on the hardware address,
which is supposed to be uniquely assigned, IP operation on the
interface MAY be continued.
Continued operation is only possible if a new IID is created. The
best procedure to follow for this will depend on the IID formation
method in use. For example, if an IID is formed by a pseudo-random
process, that process could simply be repeated.
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RFC 7136 IPv6 IID Significance February 20145. Clarification of Specifications
This section describes clarifications to the IPv6 specifications that
result from the above discussion.
The EUI-64 to IID transformation defined in the IPv6 addressing
architecture [RFC4291] MUST be used for all cases where an IPv6 IID
is derived from an IEEE MAC or EUI-64 address. With any other form
of link-layer address, an equivalent transformation SHOULD be used.
Specifications of other forms of 64-bit IIDs MUST specify how all 64
bits are set, but a generic semantic meaning for the "u" and "g" bits
MUST NOT be defined. However, the method of generating IIDs for
specific link types MAY define some local significance for certain
bits.
In all cases, the bits in an IID have no generic semantics; in other
words, they have opaque values. In fact, the whole IID value MUST be
viewed as an opaque bit string by third parties, except possibly in
the local context.
The following statement in Section 2.5.1 of the IPv6 addressing
architecture [RFC4291]:
For all unicast addresses, except those that start with the binary
value 000, Interface IDs are required to be 64 bits long and to be
constructed in Modified EUI-64 format.
is replaced by:
For all unicast addresses, except those that start with the binary
value 000, Interface IDs are required to be 64 bits long. If
derived from an IEEE MAC-layer address, they must be constructed
in Modified EUI-64 format.
The following statement in Section 2.5.1 of the IPv6 addressing
architecture [RFC4291] is obsoleted:
The use of the universal/local bit in the Modified EUI-64 format
identifier is to allow development of future technology that can
take advantage of interface identifiers with universal scope.
As far as is known, no existing implementation will be affected by
these changes. The benefit is that future design discussions are
simplified.
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RFC 7136 IPv6 IID Significance February 20146. Security Considerations
No new security exposures or issues are raised by this document.
In some contexts, unpredictable IID values are considered beneficial
to enhance privacy and defeat scanning attacks. The recognition that
the IID value should be regarded as an opaque bit string is
consistent with methods of IID formation that result in
unpredictable, pseudo-random values.
7. IANA Considerations
This document requests no immediate action by IANA. However, the
following should be noted when considering any future proposed
addition to the registry of reserved IID values, which requires
Standards Action [RFC5226] according to [RFC5453].
Full deployment of a new reserved IID value would require updates to
IID generation code in every deployed IPv6 stack, so the technical
justification for such a Standards Action would need to be extremely
strong.
The preceding sentence and a reference to this document have been
added to the "Reserved IPv6 Interface Identifiers" registry.
8. Acknowledgements
Valuable comments were received from Ran Atkinson, Remi Despres,
Ralph Droms, Fernando Gont, Eric Gray, Brian Haberman, Joel Halpern,
Bob Hinden, Christian Huitema, Ray Hunter, Tatuya Jinmei, Roger
Jorgensen, Mark Smith, Bernie Volz, and other participants in the
6MAN working group.
Brian Carpenter was a visitor at the Computer Laboratory, Cambridge
University during part of this work.
9. References9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
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